Pure gasoline used as fuel for motorcar engines includes light gasoline whose main ingredient is a hydrocarbon such as heptane, pentane and the like, heavy gasoline whose main ingredient is a hydrocarbon such as benzene and the like and medium gasoline (normal regular gasoline) which is a medium type of these two. If the heavy gasoline is used as fuel for an engine whose ignition timing and the like are controlled to be matched to the light gasoline, for example, ignition of the engine is delayed. Further, in this engine, there arises deterioration in cold startability and deterioration in driving performance, such as a breathing phenomenon and the like, and there causes an increase of toxic components in exhaust fumes due to incomplete combustion.
In the United States, European countries and the like, alcohol-blended gasoline becomes widespread as fuel for automobiles in order to reduce the amount of oil consumed. If this alcohol-blended fuel is used for an engine which is matched to the air fuel ratio of gasoline, the air fuel ratio becomes lean because the theoretical air fuel ratio of alcohol is lower than that of gasoline, and so on. For this reason, if the alcohol-blended fuel is used for a motorcar engine, it is necessary to control an actuator such as a fuel injection valve or the like on the basis of detected alcohol content in the alcohol-blended fuel and adjust the air fuel ratio, the ignition timing and the like in accordance with the detected alcohol content rate.
Therefore, for motorcar engines, it is necessary to detect which type of gasoline is used, light, medium or heavy, and the alcohol content rate in the alcohol-blended fuel. It is further necessary to control the air fuel ratio, the ignition timing and the like in accordance with the detected value.
Whether the gasoline to be used is heavy or light is correlated with its refractive index. The refractive index of the heavy gasoline is large and that of light gasoline is small. Therefore, as a liquid property sensor for gasoline, a type of sensor to measure the change of the refractive index has been developed.
Patent Document 1 discloses a liquid property sensor which inputs light from a light source into a short-period tilted-grating and analyzes spectra of the output light by a signal analyzer. With a change in refractive index of a material surrounding the grating, a transmission spectra shape in a cladding propagation mode appearing in transmission factor properties changes. In Patent Document 1, by measuring the transmission spectrum and calculating a change in shape of an envelope, the refractive index of the surrounding of the grating is detected and the liquid properties are thereby detected.
Patent Document 2 discloses a liquid property sensor which inputs light from a light source into a short-period grating. Also in Patent Document 2, with a change in refractive index of a material surrounding the grating, the change in transmission spectra shape in the cladding propagation mode appearing in transmission factor properties is used. In Patent Document 2, by measuring a change in intensity of the output light due to the change in transmission spectra shape, the refractive index of the surrounding of the grating is detected.
Patent Document 1: International Publication No. 02/44697 brochure
Patent Document 2: International Publication No. 06/126468 Brochure
In the detection using the short-period tilted-grating, as disclosed in Patent Document 1, since it is necessary to measure the spectra shape of cladding propagation mode loss peak with high resolution of wavelength, an expensive measurement device such as a light spectrum analyzer or the like is needed. For this reason, in the liquid property sensor of Patent Document 1, it is difficult to measure the refractive index only with simple detection of the quantity of light.
Further in Patent Document 1, as the tilt angle of the grating increases, the lower limit of measurable range of refractive index can be enlarged toward the lower refractive index side but the upper limit accordingly decreases. As a result, Patent Document 1 has a problem of impairing the effect of enlarging the measurable range of refractive index.
On the other hand, in the detection using the short-period grating, as disclosed in Patent Document 2, since the change in spectra shape in the cladding propagation mode loss peak is detected with the change in the quantity of transmitted light, it is possible to measure the refractive index only with simple detection of the quantity of light. Patent Document 2, however, has a problem that the measurable range of refractive index can not be sufficiently enlarged.